The key accused is one Sheikh Suhan, an active member of the TMC, who was caught on camera firing at the Sub Inspector. He has been taken to Lal Bazar Police Headquarters and is being interrogated.

A police sub-inspector was shot dead and another man injured in firing by miscreants during a clash between student activists of the Trinamool Congress and the Congress at a college here Tuesday, police said. Four accused were arrested.

Tapas Chowdhury, a sub-inspector attached to the city police's Special Branch, was shot at from close range while trying to control the clashing students in Harimohan Ghose College in the Garden Reach area of west Kolkata, police said.

Slain police officer Tapas Chowdhury's son.

"Four persons have been arrested including prime suspect Mohammad Shubhaan. Irrespective of the political affiliations, those found guilty will be punished," Home Secretary Basudeb Banerjee said.

Condemning the incident, Chief Minister Mamata Banerjee said shooting a police officer was not democracy. "Fatally shooting a police officer is not democracy. Democracy does not mean terror," Banerjee said at a meeting in Nandigram of East Midnapore district.

The violence in the college erupted over the issue of picking up nomination forms for the students' union election, police said.

"Chowdhury was hit by a bullet while trying to control the situation. He died in a hospital," he said.

A man, also injured in the firing, was in hospital, he said.

Asked about the groups which perpetrated the violence, Nesakumar said the clash took place between Trinamool Congress Chhatra Parishad, the student wing of the ruling Trinamool Congress) and Chhatra Parishad, the student wing of the Congress, who were rivals in the college election.

West Bengal Urban Development Minister Firhad Hakim, who rushed to the spot, claimed that the Congress and the Communist Party of India-Marxist (CPI-M) fomented the trouble.

Slain police officer Tapas Chowdhury's wife.

"The miscreant was earlier with the CPI-M and later he switched to the Congress. The targets were our leaders. Chowdhury took the bullet while he was trying to save them," said Hakim.

Terming the allegation as baseless, state Congress chief Pradip Bhattacharya said the incident showed how law and order had deteriorated in the state and demanded that the central government intervene.

CPI-M central committee member Md. Selim said the area had been a political flashpoint over the last few days. He alleged that some Trinamool Congress activists were injured while making bombs Monday.

Magsaysay award-winning writer Mahasweta Devi expressed sadness over the incident and said it should be ensured that there was no political tension in education.